A commonly used technique for drawing optical fibers is the double crucible technique, which is used to produce optical fibers from a molten glass couple. The finished optical fiber consists of a core surrounded by a cladding. In fabricating the optical fiber, two different materials are usually required, one for the core and one for the cladding.
The double crucible typically includes two concentric reservoirs. The inner reservoir contains material for the core and the outer reservoir contains material for the cladding. An orifice, or outlet, is drilled at the bottom of each reservoir to allow the molten glass to flow down. The optical fiber geometry is formed when the core material comes into contact with the cladding material at the outlet of the inner reservoir. The size of the optical fiber core is directly proportional to the square root of the volume ratio between the core flow and the cladding flow.
The double crucible technique is useful for the processing of materials which cannot be used in other conventional fabrication techniques such as the chemical vapor deposition (CVD) technique. Complex multi-component glass compositions are good candidates for the double crucible process. These glasses can only be obtained by melting of raw materials at high temperature or sol-gel. Another advantage of the double crucible technique is that it is a one-step process, in which the materials are transformed directly into an optical fiber. Other techniques require multiple steps. An example of this is the preform technique, in which a one-step or two-step process is required to produce a preform consisting of a core material surrounded by cladding, followed by an additional step of drawing the preform into an optical fiber.
As noted, the core size for the double crucible technique is proportional to the square root of the ratio of volume flows between the core and cladding material. Flow rates in the double crucible technique are often controlled by a simple gravity feed. In this case, volume flow is controlled by the design of the inner and outer crucibles, as well as the particular characteristics of the liquids within each of the inner and outer crucibles, including surface tension and viscosity. Flow rate at any particular time in a fabrication run is also influenced by the head pressure of liquid in each of the crucibles. The head pressure changes throughout a run as the level of liquid changes. In addition, it is possible to influence flow rates by adjustment of an outside parameter as discussed further below.
In the case of a gravity feed, molten core and cladding materials are introduced into the inner and outer crucibles, respectively. For a small core size, the ratio of core size to clad size is given by the ratio between the diameters of the inner and outer crucibles. One useful optical fiber has a core size of 4 microns within a cladding size of 125 microns. Such an optical fiber can be fabricated using an inner crucible having a diameter of 1.3 mm and an outer crucible having a diameter of 40 mm. Such a fiber can also be fabricated using other crucible sizes, so long as the ratio between inner and outer crucible diameters is the same. For example, an outer crucible having a diameter of 80 mm could be used with an inner crucible having a diameter of 2.6 mm. A gravity feed using these crucible sizes is effective with many glass compositions. However, for some compositions, particularly those subject to high surface volatilization, a tensile force at the surface of the liquids tends to produce inconsistencies in the core flow. In addition, some compositions are subject to the formation of a thin crust at the surface of the core material. This crust significantly interferes with gravity flow and tends to make gravity flow irregular and unpredictable. For these compositions, therefore, the core size is difficult or impossible to control precisely using the gravity flow method.
In many cases, it is possible to regulate liquid flow in order to control core size by applying an overpressure or underpressure to the inner crucible in order to increase or decrease core flow. This method is particularly useful for changing core to clad ratios without the need to change the ratio between the inner crucible and outer crucible. However, for glass compositions which do not give consistent results using gravity feed due to volatilization, the application of overpressure or underpressure is ineffective. This ineffectiveness results because the tensile force at the surface of the liquids continues to predominate, resulting in inconsistent flows.
Control of flow rates by regulating crucible size also has related costs and results in the inconvenience associated with the need to change crucible sizes in order change flow rates and core sizes.
There exists, therefore, a need in the art for a technique for double crucible drawing of optical fibers which will produce acceptable results for a variety of core materials, including core materials which are subject to tensile forces at the surface of the liquids, and which provides a means for changing flow rate without a need to use different or redesigned crucibles.